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Modern large calcareous Foraminifera in shallow tropical seas have rapid growth rates in comparison with calcareous Foraminifera in temperate waters. Tropical species commonly exceed dimensions of 15 mm and volumes of 200 mm3 in two years of growth—a rate 50 to 100 times that of most temperate species.

Differences between tropical and temperate species include growth form, life history, and en¬vironmental parameters, as well as several intracellular features. Many tropical forms have distinc¬tive growth geometries and internal construction that place a greater percentage of protoplasm near the outer edge of the shell, such as flattened or discoidal shapes, annular or equatorial chamberlets, labyrinthic partitions of the shell wall, clear thin areas of shell wall between partitions, and finger¬like protruberances. Tropical species grow throughout the year in water that usually is considerably warmer and more constant in temperature and in sunlight that is more intense, in contrast to the seasonal growth of temperate species. These environmental differences contribute to some of the differences in growth, but it is doubtful if they contribute to all the differences.

Intracellular differences between tropical and temperate forams show that both have numerous symbionts within their cytoplasm, but the temperate species have moneran (cyanellae) blue-green algal “chloroplasts” and the large tropical forms have closely packed zooxanthellae—dinoflagellates similar to those found in tropical reef-dwelling corals and in some other invertebrates. The high rates of growth in tropical forams are commonly accompanied by relatively large amounts of mag¬nesium (12 to 14% in some species) in the calcite lattice structure of the shell.

The place of Foraminifera in the ecology of reefs is still poorly understood; however, they con¬tribute about 10 to 15% of the calcareous sediment in and directly adjacent to many reefs, and their shell adaptation for symbiotic relationships with zooxanthellae suggests that they make a more important contribution to reef ecology than commonly is attributed to them.

The development and evolution of large calcareous forams in the late Mesozoic and Cenozoic closely coincides with the great increase in diversity of dinoflagellates, the group to which zooxan¬thellae belong. This suggests a close symbiotic relationship and interlocking evolutionary history of many organisms that are part of the Holocene hermatypic coral-reef ecosystem studied. The late Paleozoic Fusulinina probably had a symbiotic relationship with different photosynthetic auto¬trophs, as may also be the case for other reef and bank organisms of that time. Climatic fluctua¬tions late in the Paleozoic may have caused an extinction of the symbionts which resulted in the extinction of many shallow-water marine invertebrates.

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